This invention relates to a semiconductor memory, and an effective technology for applying the invention, for example, to a dual-port memory (multi-port memory) having a sense amplifier and the like.
There is available a dual-port memory having both a random access port and a serial access port. The dual-port memory is often used as, for example, a picture frame buffer memory for displaying characters, figures, or the like on the face of a CRT(cathode-ray tube). For this purpose, the random access port and the serial access port of the dual-port memory are provided with a plurality of data input/output terminals and data input/output circuits to which memory data are input/output every 4 bits or 8 bits, respectively. Each of the bits is made to correspond to each of color factors of displayed pictures.
The dual-port memory is reported on, for example, pp 243 through 264, "Nikkei Electronics" dated Mar. 24, 1986 published by Nikkei McGraw Hill Co., Ltd.
In the case where the dual-port memory is used as a picture frame buffer memory, for example, when the background color of the displayed picture is altered, the so-called clear function by which stored data in all memory cells corresponding to a specific color factor of the stored picture is rewritten into the same data at a high speed is said to be effective. Also, it is desirable for the clear function to allow the serial output operation based on the serial access port to be executed while the picture is displayed, in other words, without a break of the above operation.
The above-reported dual-port memory is provided with a random access port which is randomly accessible irrespective of a state of operation of the serial access port. The random access port is prepared with the so-called write per bit function by which maskable write is executed in accordance with the mask data supplied in advance from the outside. The use of the write per bit function allows the clear operation corresponding to the color factor to be realized. Since the clear operation using the write per bit function can not, however, help by being executed, for example, every 4 bits or 8 bits, a considerable amount of time is needed before a series of picture data are rewritten.